The present invention relates to a fail-safe system for a four wheel steering system of a vehicle.
There have been proposed various four wheel steering systems. Some recent examples are disclosed in commonly-assigned, copending U.S. patent applications: (i) U.S. Ser. No. 07/284,414; filed on Dec. 14, 1988 (corresponding to a European Patent Application No. 88 121 139.5), (ii) U.S. Ser. No. 07/305,023; filed on Feb. 2, 1989, now U.S. Pat. No. 4,934,474 (corresponding to a European Application No. 89 101 947.3), and (iii) U.S. Ser. No. 07/343,379; filed on Apr. 26, 1989 (corresponding to a European Application No. 89 107 570.7).
Many of the four wheel steering systems employ a hydraulic actuator for steering the rear wheels, an electromagnetic control valve for controlling an oil pressure supplied to the hydraulic actuator, and an electronic controller for controlling the control valve. In a four wheel steering system of such a type, however, a failure in an electrical circuit easily impairs the stability of the vehicle by causing an abrupt change in vehicle behavior, and arouses an unnatural feeling in the driver. For example, a failure in the electronic controller or a breakage of a solenoid of the electromagnetic control valve makes the electromagnetic valve inoperative, so that the oil pressure in the actuator is immediately reduced to zero. As a result, the rear wheels are abruptly returned to a straight ahead position by the action of a center spring of the actuator, and the behavior of the vehicle is changed abruptly.